animateScrollTo.js

// easing functions http://goo.gl/5HLl8
Math.easeInOutQuad = (t, b, c, d) => {
  t /= d/2;
  if (t < 1) {
    return c/2*t*t + b
  }
  t--;
  return -c/2 * (t*(t-2) - 1) + b;
};

Math.easeInCubic = (t, b, c, d) => {
  const tc = (t/=d)*t*t;
  return b+c*(tc);
};

Math.inOutQuintic = (t, b, c, d) => {
  const ts = (t/=d)*t;
  const tc = ts*t;
  return b+c*(6*tc*ts + -15*ts*ts + 10*tc);
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (() => window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || (callback => { window.setTimeout(callback, 1000 / 60); }))();

function scrollTo(to, callback, duration) {
  // because it's so fucking difficult to detect the scrolling element, just move them all
  function move(amount) {
    document.documentElement.scrollTop = amount;
    document.body.parentNode.scrollTop = amount;
    document.body.scrollTop = amount;
  }
  
  function position() {
    return document.documentElement.scrollTop || document.body.scrollTop || document.body.scrollTop || window.pageYOffset;
  }
  
  const start = position();
  const change = to - start;
  
  let currentTime = 0;
  const increment = 20;
  
  duration = (typeof(duration) === 'undefined') ? 500 : duration;
  
  const animateScroll = () => {
    // increment the time
    currentTime += increment;
    // find the value with the quadratic in-out easing function
    const val = Math.easeInOutQuad(currentTime, start, change, duration);
    // move the document.body
    move(val);
    // do the animation unless its over
    if (currentTime < duration) {
      requestAnimFrame(animateScroll);
    } else {
      if (callback && typeof(callback) === 'function') {
        // the animation is done so lets callback
        callback();
      }
    }
  };
  animateScroll();
}